Lipoprotein lipase (LPL) is an enzyme present on the surface of capillary walls. LPL acts to hydrolype triglycerides into free fatty acids (FFA) and monacylglycerol. In adipose tissue, LPL (ATLPL) provides FFA for storage in the adipocyte. In skeletal muscle, LPL (SMLPL) provides FFA for oxidation. In hibernating animals, SMLPL is relatively higher in summer than winter (promoting fat oxidation) while the opposite is seen with ATLPL (relatively higher in the winter promoting fat storage ). Preliminary data show this may also be occurring in humans. This 'multi-season' patient controlled study is designed to determine if such variation occurs in normal weight humans. There have been 13 subjects studied both during summer of 1995 and the winter of 1995-1996. They have underwent muscle and adipose biopsies of LPL, assessment of cardiorespiratory fitness by VO2 max, and determination of body composition by hydrostatic weighing. An additional 5 subjects have begun the study in the winter of 1996 (with the procedures described above), and will complete the study in the summer of 1996. Preliminary data from the 13 subjects completing the study reveal a four-fold increase in ATLPL from summer to winter (p<0.01) with a non-significant trend upward in SMLPL. These are despite the lack of change in body composition (BMI and hydrostatic weighing), fitness (VO2 max) and dietary intake two days preceeding the study. Data from the winter to summer subjects are pending.